The invention relates to a method for regulating a magnetic deflection system for a particle radiation optical device, particularly an electron beam writer, in which the current flowing through the deflection coils is measured and a control signal for the readjustment of the coil current is generated by means of comparison with a command value. The invention further relates to a device for implementing this method.
In order to achieve short write times with an electron beams writer in the exposure of semiconductor wafers, a rapid and high-precision deflection system for the electron beam is required in addition to a high beam current density and a rapid data processing. Given the employment of magnetic deflection systems, the magnetic field is determined by measuring and regulating the coil current in deflection coils. The coil current can be simply measured with sufficient precision in order to achieve the required precision of the magnetic fields from 10.sup.-4 through 10.sup.-5 even given the small magnetic fields of a few 10.sup.-4 Tesla hereby occurring. Rapid changes of magnetic field, however, are influenced by eddy currents which are induced in surrounding, conductive components. Under certain conditions, thus, the magnetic field cannot quickly enough follow the command value. A current regulation with additional differential behaviour is only admissible insofar as no overshoot (or: ringing) occurs beyond the allowed tolerances by so doing.
A control system which is based on a direct measurement of the magnetic field could avoid these problems. In so doing, however, there is difficulty in statically and dynamically measuring the small magnetic fields with the precision required here.
The greatest possible employment of nonconductive materials, ferrite shields and the observance of the greatest possible intervals from conductive surfaces only produces limited success.